1. Field of the Invention
The present invention pertains to heterojunction devices and particularly to CdS - Cu.sub.2 S thin film solar cells.
2. Description of the Prior Art
Some effort has been made in the past to produce a backwall CdS -- Cu.sub.2 S thin film solar cell employing a transparent substrate anticipating advantages over conventional frontwall CdS -- Cu.sub.2 S solar cells. For the purposes of this specification, the term "frontwall" refers to devices illuminated through the Cu.sub.2 S layer to the junction while "backwall" refers to devices illuminated through the CdS layer to the junction. In a frontwall arrangement, which is the present standard type of cell construction, a layer of CdS is deposited on a metallic substrate and then a layer of Cu.sub.2 S is formed on the CdS. A metallic grid is then applied to the surface of the Cu.sub.2 S, and finally transparent protective coating is applied. In a particular configuration of a backwall CdS -- Cu.sub.2 S thin film solar cell, the Cu.sub.2 S is adjacent to the substrate and the CdS is then deposited on the Cu.sub.2 S. In this backwall structure a grid is also attached, but in this case it is attached to the thicker, more conductive CdS layer which permits a coarser, less expensive grid to be used. Furthermore, in the backwall structure a protective coating is not needed since the Cu.sub.2 S layer is not vulnerable to the atmosphere as a result of it being sandwiched between the substrate and the stable CdS layer.
Unfortunately, these and other hoped for advantages of the backwall structure described above have not been fully realized since past attempts to produce such a backwall structure failed to reproduce the essential epitaxially formed Cu.sub.2 S -- CdS heterojunction. The present invention solves this problem, and provides a method for producing an epitaxial heterojunction device.